Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens, and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor and a display device positioned partially or fully behind the touch sensor so that the touch-sensitive surface can cover at least a portion of the active area of the display device. The display device can include technologies such as liquid crystal displays (LCDs), organic light emitting diode (OLED) displays, etc. OLEDs, for example, can provide a flat or flexible display in a relatively thin package that can be suitable for use in a variety of portable electronic devices. In addition, OLED displays can display brighter and more vibrant images in a thinner and lighter package compared to LCD displays, making them suitable for use in compact portable electronic devices.
Sensing a proximate object using one or more capacitance-based (e.g., self- and/or mutual capacitance) touch sensors can provide an input modality for an electronic device. Recent advancements in touch sensor technology have allowed capacitance-based touch sensors to perform at higher speeds and at higher touch resolutions than was previously possible. Some applications, such as user identification, may desire a higher sensitivity and/or resolution than capacitance-based touch sensing.